Prepended an 'if' to remove error bell on osx systems.

* Don't override user defined styles
* Better modularisation of highlighters
* Allow to define which highlighters are activated
* Allow to define the order in which they are defined
* Minor performance optimizations
* Fixed some variables leak
* Improve documentation
* Brackets highlighter: use ZSH_HIGHLIGHT_STYLES instead of a specific array

Feel free to add more test data !

Signed-off-by: Julien Nicoulaud <julien.nicoulaud@gmail.com>